An option you may want to look for is a Gear Vendors overdrive unit. It installs after the transmission, and gives you an extra "gear" for steady flatland cruising. Get into the hills, kick out the OD, and let the engine pull up on revs. They make these things for medium duty applications, so you should be able to find something that will get you closer to your goals. Caveat is though, that they are NOT cheap.

As cool as I think the GV is (and it IS cool), I don't think they are rated for the GVWR a bus has to deal with. On top of that, finding a way to mate one to an Allison could be difficult at best. There are, however, similar auxilary transmissions that are somewhat common in MDT's.

Busone, what kind od setup do you have to get 13 mpg? Is your powertrain stock or have you made a few mods?

Wolfman, I can get a rebuilt pumpkin from the International dealer for $1400.00 plus 4 hours of labor at $85/hour. That may be cheaper then the OD unit and modding the drive shaft.

My thoughts are, it takes horsepower to climb mountains, and with 210 hp, I'm only going to get 30 to 35 mph on a long 6 percent grade, be it in 2nd, 3rd, or 4th gear, depending on the axle ratio. Therefore I may as well cruise the flats at a comfortable ( and quiet ) RPM, plus get the extra MPG bonus by not taching out my engine. Perhalps a 3:42 ratio may cause too much strain. I may re-consider a 3:70 ratio, which may give me the best of both worlds. I will admit, the 4:44 ratio is great for city stop and go traffic, but I plan to cruise the big roads. What say Ye?

16 mpg is a VERY optimistic prediction of mileage with the higher gears versus my stock 8 mpg. You will get twice the mileage. That means that I have to spend twice as much to go the same distance...simple enough. At $2.25 per gallon for diesel, I will drive buy 800 miles on that $1700 you just spent on the conversion. That gets me 6400 miles. That might not seem like much, but that's what you would have to drive just to break even on the swap assuming that astronomically high 16 mpg. At a more realistic 12 mpg you will have to drive over 8000 miles to break even. Start factoring time, unforseen setbacks, etc and the swap starts to seem like less of a finacial savings. I will stand my ground that his ratio is already low enough. Since the difference in RPM is not important to the longevity of the engine, the only issue I see is mileage. A better investment of that $1700 would probably be a WVO system or other alternative energy source. It is just like the people trading in their SUV's for a Kia to save on fuel while taking an enormous hit in depreciation. As great as thing sound up front, you really need to look at the actual return on investment. Just my $.02 (jumps off soapbox)

I just bought a 1993 ten window Intl. 210 HP aftercooled DT466 (all mechanical) with an MT643. My only trip so far has been driving it home 200 miles from Seattle through rush hour traffic. I got an honest 10mpg on that miserable trip driving a bus I was not familiar with and constant slowing and speeding up. I'm pretty confident I will be able to squeeze 12MPG out of her on 55-60 mph highway cruises. I was turning 2200 @55mph and 2400@60mph. It seemed to find a sweet spot around 58mph which will be a good cruising speed for my travels in my retirement.

Based on my prior experience with boats and OTR trucks a mechanical diesel likes to be run at 3/4 to 7/8 of full power (think boat's cruise speed). It's different with electronic diesels. My last engine, a Cat C-7 with an Allison 3060 could be safely run anywhere from its 1340rpm torque peak up to its powered redline @2500rpm. That engine had an unpowered redline (downhill coasting) of 2950rpm. This DT466 went up to 67mph with a little bit left but I was driving a newly purchased vehicle so I wasn't going to push it too hard. I think it's safe to say the mechanical DT466 can run 2500rpm all day with occasional forays into the 2900rpm range (governor allowing).

I think anyone would be very hard pressed to get 16 MPG from a DT466 in a 20k lb. vehicle